The level of carbonation is a key property of a variety of soft drinks, as well as of some other beverages, such as champagne and seltzer water. The carbonation level is one of the principal attributes that relates to consumer acceptance and preference, thus, it is carefully monitored during beverage production, and in the trade.
The measurement and control of carbonation level, however, is not very easily accomplished primarily because the measurement has to be performed at pressures considerably higher than ambient; pressures in beverage bottles and cans are typically several atmospheres above atmospheric, depending on the temperature of the beverage.
Carbon dioxide (CO2) is relatively highly soluble in water compared to gases such as nitrogen (N2) or oxygen (O2), however, in order to produce acceptable soft drinks, or champagne for that matter, the solvent (primarily water (H2O) has to be supersaturated with CO2.
Now, CO2 solubility in water (or of any gas in any solvent) is governed by Henry's Law, which simply states that the solubility of the gas is directly proportional to the PARTIAL PRESSURE of the gas above the liquid. The proportionality constant relating the CO2 partial pressure to the solubility is called the Henry's Law constant, and is considerably temperature dependent. In order to achieve the required carbonation level of soft drinks, the CO2 partial pressure has to be raised to several atmospheres. Indeed, the release of this pressure from the container at the time of beverage consumption causes the sudden release of CO2 in the form of bubbles, which is believed to be related to the tingling, somewhat painful, never-the-less pleasing taste of the beverage.
It is a relatively simple matter to measure the CO2 level in a pure liquid in any reasonably well equipped analytical laboratory. For example, one may titrate the CO2 with a base (utilizing its acidic nature), or measure its Infra Red (IR) absorption, or separate and measure the gas from the liquid by means of a gas chromatograph, just to name a few. In an industrial environment, however, some of these methods are not suitable for a number of reasons, such as cost, speed, ease of operation, just to name a few.
Historically, the beverage industry relied on the method, implied by the above mentioned Henry's Law, namely, the measurement of temperature (T) and pressure (P). Since there is a direct relationship between the partial pressure of CO2 and its beverage concentration, at any given temperature, it is a simple matter to develop a look-up table of carbonation levels; one axis of this table is the temperature, the other the pressure, and the table values represent the carbonation levels in some convenient units. An early example of such a chart, widely used by the beverage industry, was the Heath Chart.
The CO2 content of the beverage is typically expressed in units of VOLUMES (VOL), which is a misnomer; it is actually a dimensionless unit, namely, it is the ratio of the volume of gas dissolved per unit volume of liquid. Of course, gases are compressible, and the gas volume is typically expressed at 0 degrees Celsius. Thus, for example, a beverage containing 3 VOLUMES of CO2 at say 25 deg C., would contain 3 liters of CO2 (expressed at 0 deg C.) in 1 liter of beverage at 25 deg C.
A typical measurement is made, by placing the beverage container into a puncturing device, shaking the sample to facilitate the equilibration of CO2 between the headspace and the beverage, followed by the measurement of P and T. The value of the CO2 concentration is then found from the look-up table (or calculated from an equation which expresses the temperature and pressure dependence of Henry's Law).
The Heath chart has been improved upon over the years, and different charts have been developed for different beverages, as the solubility of CO2 in them varies, primarily due to dissolved sugar.
There is, however, a rather important problem with the whole method of P and T measurement, and the CO2 VOLUMES obtained from them. It was mentioned earlier that the concentration of CO2 is related to the PARTIAL PRESSURE of CO2. The total pressure in a beverage bottle is the sum of the partial pressures of all the gases present. Water vapor contributes slightly, but it can be easily subtracted from the total pressure, as it is well known over a considerable temperature range. A much larger problem is the partial pressure contributed by the small amount of entrapped AIR in the bottle. Even though the amount of air in the bottle is small, its partial pressure is rather large due to its small solubility in (sugar) water. The amount of air in a bottle can be measured, with difficulty, but its contribution to the partial pressure is a complicated function of beverage and headspace volume, as well as of temperature.
Rather than correcting for the presence of air, a relatively clever trick is used to minimize its effect. Prior to CO2 measurement, the headspace pressure is quickly released; a process called SNIFTING. The rationale is that, since the solubility of air in the beverage is small, the air is primarily in the headspace, thus SNIFTING results in the loss of most of the air from the container. Subsequent measurement of the pressure can be taken to be that of CO2 only. This assumption is fairly good, and rather precise measurements of CO2 concentration can be made (within 1 to 2%) if the snifting is done with care. However, as anyone who opened a soft drink container knows, as the container is opened, usually some CO2 escapes as well, as can easily be seen from the evolution of bubbles from the beverage. Thus, more than just the air escapes. The amount of CO2 that escapes depends on a number of variables: beverage type, container type, temperature, history of the sample, relative headspace, and beverage volume, etc. Thus, the error introduced by snifting can easily be as large as 10% of the measurement. The analyst is confronted with the error caused by the presence of air, or the error caused by the snifting procedure.
A number of more sophisticated instrumental methods have been transplanted from the analytical laboratory to the beverage production platform, for example methods using IR absorption, and gas chromatography. Also, an interesting procedure was suggested using gas permeable membranes (Szerenyi et. al. U.S. Pat. No. 4,517,135). None of these methods have found wide acceptance in the beverage industry for a variety of reasons, notably cost, analysis time, and ease of universal adaptability; the beverage industry still prefers the simple P and T measurement method despite its shortcoming, principally due to the confounding effect of air.